Comparative study of the key enzymes and biochemical substances involved in the energy metabolism of Pacific white shrimp, Litopenaeus vannamei, with different ammonia-N tolerances.

Pacific white shrimp, Litopenaeus vannamei (9.38 ± 0.17 cm, 10.08 ± 0.35 g), with different ammonia-N tolerances were exposed to NH3 (1.61 mg/L) for 192 h, and the levels of key enzymes and biochemical substances involved in energy metabolism were compared to assess the role of the regulation of energy metabolism on the shrimp's adaptation to ammonia-N stress. Higher ammonia-N tolerance in the shrimp (Tolerance group) was achieved through nutritional fortification, whereas shrimp that were not nutritionally fortified comprised the Control group. The mortality rates in the Control and Tolerance groups at the end of the period of ammonia-N stress exposure were 64.44% and 40.00%, respectively. Within 1 h of exposure to ammonia-N stress, the glucose concentration in both groups declined rapidly, and no significant difference was detected between the two groups. In general, the triglyceride and cholesterol concentrations in the Control group were higher than those in the Tolerance group, and accumulations and/or fluctuations in these metabolites to varying degrees were observed. The Tolerance group presented higher phosphofructokinase (PFK) and pyruvate kinase (PK) activity compared with the Control group from 1 to 48 h of exposure to ammonia-N stress, whereas the opposite result was observed from 96 to 192 h. Similarly, during exposure to ammonia-N stress, the Tolerance group showed higher and lower lactate dehydrogenase (LDH) activity than the Control group from 1 to 24 h and from 48 to 92 h, respectively. In addition, compared with the Control group, the shrimp in the Tolerance group exhibited higher succinate dehydrogenase (SDH) activity, especially from 48 to 192 h of exposure to ammonia-N stress. The results of this study suggest that anaerobic carbohydrate (in the early stage) and aerobic metabolism (in the late stage) plays an important role in the shrimp's response to ammonia-N stress. In addition, maintenance of the normal operation of lipid metabolism is equally important for improving the tolerance of L. vannamei to ammonia-N stress.